Movements.The movements allowed in this articulation are limited to rotatory movements of the head of the radius within the ring formed by the annular ligament and the radial notch of the ulna; rotation forward being called pronation; rotation backward, supination. Supination is performed by the Biceps brachii and Supinator, assisted to a slight extent by the Extensor muscles of the thumb. Pronation is performed by the Pronator teres and Pronator quadratus.

Middle Radioulnar Union.The shafts of the radius and ulna are connected by the Oblique Cord and the Interosseous Membrane.

The Oblique Cord (chorda obliqua; oblique ligament) (Fig. 329).The oblique cord is a small, flattened band, extending downward and lateralward, from the lateral side of the tubercle of the ulna at the base of the coronoid process to the radius a little below the radial tuberosity. Its fibers run in the opposite direction to those of the interosseous membrane. It is sometimes wanting.

The Interosseous Membrane (membrana interossea antebrachii).The interosseous membrane is a broad and thin plane of fibrous tissue descending obliquely downward and medialward, from the interosseous crest of the radius to that of the ulna; the lower part of the membrane is attached to the posterior of the two lines into which the interosseous crest of the radius divides. It is deficient above, commencing about 2.5 cm. beneath the tuberosity of the radius; is broader in the middle than at either end; and presents an oval aperture a little above its lower margin for the passage of the volar interosseous vessels to the back of the forearm. This membrane serves to connect the bones, and to increase the extent of surface for the attachment of the deep muscles. Between its upper border and the oblique cord is a gap, through which the dorsal interosseous vessels pass. Two or three fibrous bands are occasionally found on the dorsal surface of this membrane; they descend obliquely from the ulna toward the radius, and have consequently a direction contrary to that of the other fibers. The membrane is in relation, in front, by its upper three-fourths, with the Flexor pollicis longus on the radial side, and with the Flexor digitorum profundus on the ulnar, lying in the interval between which are the volar interosseous vessels and nerve; by its lower fourth with the Pronator quadratus; behind, with the Supinator, Abductor pollicis longus, Extensor pollicis brevis, Extensor pollicis longus, Extensor indicis proprius; and, near the wrist, with the volar interosseous artery and dorsal interosseous nerve.

Distal Radioulnar Articulation (articulatio radioulnaris distalis; inferior radioulnar joint).This is a pivot-joint formed between the head of the ulna and the ulnar notch on the lower end of the radius. The articular surfaces are connected together by the following ligaments:

The Volar Radioulnar.

The Dorsal Radioulnar.

The Articular Disk.

The Volar Radioulnar Ligament (anterior radioulnar ligament) (Fig. 334).This ligament is a narrow band of fibers extending from the anterior margin of the ulnar notch of the radius to the front of the head of the ulna.

The Articular Disk (discus articularis; triangular fibrocartilage) (Fig. 336).The articular disk is triangular in shape, and is placed transversely beneath the head of the ulna, binding the lower ends of the ulna and radius firmly together. Its periphery is thicker than its center, which is occasionally perforated. It is attached by its apex to a depression between the styloid process and the head of the ulna; and by its base, which is thin, to the prominent edge of the radius, which separates the ulnar notch from the carpal articular surface. Its margins are united to the ligaments of the wrist-joint. Its upper surface, smooth and concave, articulates with the head of the ulna, forming an arthrodial joint; its under surface, also concave